In most air conditioning and air supply systems, it is generally desirable to filter some of the foreign materials such as dust or dirt particles out of the air which is being supplied to the system. The amount of foreign material desired to be extracted from the incoming air or correspondingly the amount of foreign material which is tolerable in the particular air conditioning or supply system will dictate the particular type of filtering media which must be used. If only relatively large particles need to be eliminated, a filter utilizing wire screen media may be adequate, whereas if relatively fine dust particles must be removed from the incoming air, the filtering media must normally consist of such materials as cloth-backed cotton, polyurethane foam, glass fiber media, spun nylon and the like. It is the filtration of these finer particles with which the filter herein described is concerned.
A common problem in the filtration of air is that the foreign material extracted from the incoming air accumulates and the filtering media eventually becomes sufficiently clogged or loaded with dirt that its resistance to air flow becomes excessive. When this occurs, the media must be either replaced or cleaned. This problem is aggravated in the case of filter media of the types described which are effective for the finer dirt particles. With most types of such media, it is impractical to clean the same and it has therefore become relatively common to use inexpensive disposable filters.
Filtering materials which provide reasonable efficiency in the removal of the relatively finer foreign particles have relatively high resistance to the flow of air therethrough. As a result, it is generally desirable to provide an area filtering media for the air to flow through which is greater than the area of the air inlet duct to the air conditioning or supply system. To accomplish this, it is common practice to corrugate or pleat the filtering media thereby increasing the area of filtering media through which the air may flow. Most types of filtering media which are effective against finer dirt particles are not sufficiently rigid, however, to be self-supporting in this corrugated or pleated configuration, with the result that some apparatus must be supplied to support the filtering media.
More efficient filtering of relatively fine foreign particles and increased filter life can be obtained with the use of high loft micro glass fibers as a filtering media than with the thinner, higher density water-laid glass media due to the increased dust holding capacity of the high loft material which allows the filter to hold more particulate matter without a significant increased pressure drop. Because of the lack of self-support and the relatively high loft of this type of media, great difficulty is encountered in attempting to utilize such media in a pleated configuration.
The efficiency of the filter can be further increased by increasing the depth of the pleats and consequently the filter assembly to correspondingly increase the amount of filtering media through which the air may flow. However, such a modification renders the problem of supplying supporting apparatus for a filter assembly incorporating high loft filtering media even more acute.
In supporting the filtering media the supporting apparatus defines to a large extent the inlet and outlet openings of the filter and consequently has a great effect on the filter's efficiency. With the thinner, higher density material, a plurality of corrugated separators can be employed which extend parallel to and between the pleats to maintain the desired configuration of the pleats. However, if such separators were used with the high loft filtering media, the separators would press into the softer material distorting the pleated configuration and interfere with the air flow therethrough.
Another means of supporting the pleats which has been employed in filters using the denser and thinner media is the use of wire grids. While a wire grid may be adequate as a sole supporting member in relatively narrow filter assemblies, they have proved unsatisfactory for use in supporting deep pleats of high loft filtering media as they lack the structural strength to maintain the configuration of a pleat of about 12 inches in depth and if the individual wires are too closely spaced they tend to overly compress the filtering media held therebetween and thereby largely eliminate the advantage of the high loft filtering media. If the grid pattern were enlarged to avoid compression of the filtering media and constructed of a sufficient gauge to provide the necessary structural strength to maintain the pleated configuration during use of the filter assembly, the grid supporting structure could not be economically secured to the filter media without tearing the media and severely damaging the assembly.
Another means which has been used to support pleated filtering media is to suspend the media, which is provided with a relatively light gauge wire web backing, in a pleated configuration in a collapsible frame. The frame and media are then inserted into a rigid housing which includes a sinuous wire support which is disposed on one side of the media. Unlike the methods discussed above, this configuration is suitable for use with deep pleats of high loft filtering media. However, it has several shortcomings when so used. To facilitate assembly, the wires comprising the support are not closely spaced as in a wire grid. Consequently, some billowing of the media occurs during use which obstructs the air flow therethrough. In addition, the use of a one-sided support exclusive of the thin wire backing which merely gives a slight element of rigidity to the media, does not provide sufficient control over the configuration of the individual pleats and consequently restricts the number of pleats which can be incorporated in a given area. Due to this lack of control, additional pleats would cause continual distortion of the air inlet and outlet openings defined by the pleats and a drop in efficiency. Accordingly, the number of pleats and area of filter media is reduced. The combination of reduced media and less support produces a less efficient filter.